Teresa Burt has voted Democrat all her life, but can't quite explain why she won't be voting for Barack Obama on Nov 4.

"I just don't trust him on certain things. He is just not who I want to run this country," said the mother of two who as she waited on tables at the Ben Evans diner.

"If I vote, it will be McCain. And I know a lot of Democrats who won't vote for Obama too."

Volunteers at the Obama headquarters two blocks away in Portsmouth, Ohio, are convinced of the explanation to the vague responses they elicit in the state: race. They hear it often on the phone.

"Some people are openly racist, they use the N-word, they are very clear they don't want to see a black man in the White House," said Jeanette Langford, taking a break from working through her call lists.

"Others aren't even aware that race is what's holding them back from supporting Barack. But what else is it? They never mention issues, it's always things like 'I can't trust him', or 'he is not the right leader for me'. When they talk about not supporting McCain, it's always issues."

Mr Obama does have other problems, such as his lack of experience, his failure to produce snappy, winning ideas for the troubled economy, and his difficulty empathising with struggling families as effectively as other politicians.

Randy Basham, the Democratic Party chair in Portsmouth's Scioto County, has said race was Mr Obama's biggest obstacle. Ted Strickland, the state's Democratic governor, called the issue the "elephant in the room" of the campaign.

"It's what everybody is thinking about but nobody is talking about," he said.

Mr Obama's mixed race fuelled interest in his candidacy early on at home and abroad. For some Americans, it remains a significant reason for supporting him, while for others the novelty has worn off.

In places like southern Ohio, however, Democrats fear that his race - Mr Obama was born to a black Kenyan father and a white American mother - is denying him what should be an unassailable lead in the polls.

With a Republican president presiding over an unpopular war and calamity in the economy, campaign workers sense that in the most depressed region of a depressed state their candidate should be several notches above the average four-point advantage he currently holds over Republican Senator John McCain in most polls.

The state's "three Cs" - Cincinatti, Cleveland and Columbus - vote Democrat and wealthier rural areas vote Republican, leaving the poor south holding the balance of power in a large swing state that voted twice narrowly for Bill Clinton and twice narrowly for George W Bush. A loss in Ohio by a few points could lose Mr Obama the White House.

Obama began a two-day visit the area on Thursday, speaking at a college in Portsmouth, a struggling town on the banks of the Ohio River where a giant Wal-Mart has replaced the steel works.

In the insular, clannish, "guns and god" culture of the wooded foothills of Appalachia, large populated two by descendants of émigrés from the segregated south, the colour of a person's skin still matters to a minority.

There have been reports in Portsmouth of Obama signs on front lawns being vandalised or stolen, while a friend of one volunteer who has an Obama sticker on her car said she was jeered when she came out of a shop.

The volunteers at Obama HQ estimated that they heard two or three explicitly or implicitly racist comments out of 20 calls.

"It's tough going, but this is southern Ohio," said Mrs Langford, who is African American and has lived in Portsmouth most of her life. "But it upsets me we still are living with this after all this time."

Her colleague Emily Cobb-Thomas, who is white, said: "This is the Appalachians. There are a lot of rednecks here and they are born and raised to look down black people."

Given the possibility that a minority of those negative responses are Democrats, does the campaign really stand to lose many votes to racism?

"The problem is that we don't know how many people are thinking these things and not saying them. It can be scary because they could bring our man down," said Mrs Langford.

Skip Kyle, who owns a sign shop next door to the Democratic office, is one of those unashamed to say race is a factor in his decision. "I would be a hypocrite to say it wasn't. I think Obama is an angry man.

"I just do not agree with black culture in this country, the violence and the drug use," said Mr Kyle, a Republican who has voted Democrat in the past.

Others said it helped that Obama was only half black, and had been raised by his white mother and her parents.

"I don't know why he says he is black. He was raised by whites - and you know your raising and your education, that all counts," said Patrick Coll, a construction worker who is paid $9 an hour.

Like many voters in Portsmouth, Mr Coll is still undecided. "Race doesn't matter to me," he said. "I just want to hear something real clear and absolute from either candidate about how we are going to get out of this mess."